Reversal of magnetic core polarization



Oct. 21, 1969 RO ET AL 3,474,426

REVERSAL 0F MAGNETIC CORE POLMIIZATION 2 Sheets-Sheet 1 Original Filed July 5, 1955 Fig.2

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2 Sheets-Sheet 2 D. s. RODBELL ET AL REVERSAL OF MAGNETIC CORE POLARIZATION Oct. 21, 1969 Original Filed July 5 3Q t Ext 25 amt RES Their Attorney- United States Patent 3,474,426 REVERSAL OF MAGNETIC CORE POLARIZATION Donald S. Rodbell, Burnt Hills, and Charles P. Bean,

Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Continuation of application Ser. No. 519,992, July 5, 1955. This application Feb. 13, 1968, Ser. No. 707,379 Int. Cl. Gllb 5/00 U.S. Cl. 340-174 Claims ABSTRACT OF THE DISCLOSURE A method is disclosed for reducing the time required for reversing the magnetic polarization of ferromagnetic materials by the application of a driving field having a Wave shape comprising an initial high amplitude for a short time to initiate the reversal of magnetization of the material and then decreasing the amplitude to a much lower value whereby the reversal is completed, and an apparatus for accomplishing the result.

This application is a continution of copending Ser. No. 519,992, filed July 5, 1955 and now abandoned and assigned to the assignee of this invention.

This invention relates to the reversal of magnetic polarization of soft magnetic materials and, more particularly, to the reduction in the time hitherto required to reverse the magnetic polarization of soft magnetic materials having substantially rectangular hysteresis loop characteristics.

In the following disclosure the term soft magnetic materials will be understood to refer to those materials which acquire a large fraction of their total potential magnetization when exposed to a relatively weak magnetic field, for example, a field of the order of and frequently less than one oersted. Further, the term rectangular hysteresis loop will be understood to refer to the shape or configuration of the direct current hysteresis loop characteristic of such materials as will be discussed in more detail later.

Soft magnetic materials are commonly used in electrical and electronic apparatus and circuits as cores which may be reversibly magnetically polarized by induction. This is usually accomplished by subjecting all or part of the core to a magnetic field provided by passing an electric current through a coil which may be wound around at least part of the core. As is well known, if the electric current is large enough a sufficiently large magnetic field will be provided to magnetically polarize or magnetize the core. The direction of polarization of the core is determined by the direction of the current through the coil according to widely known principles governing electromagnetic behavior.

When the impressed magnetic field is removed by interruption of the current in the coil, the soft magnetic core exhibits a phenomena known as remanence or residual magnetism in that after a magnetizing field is removed the core remains magnetically polarized or magnetized in the direction of the formerly impressed field.

In certain apparatus or circuits it is desirable to reverse the magnetic polarization of these cores. This is accomplished by subjecting the core to a magnetic field having an opposite polarization and may be done, for example, by merely reversing for suificient time the direction of the electric current through the coil referred to previously. However, it is to be appreciated that reversal of the field may be accomplished by other functionally equivalent means such as a different coil, for example. After the magnetic polarization has been reversed, the reversing field may be removed and again it will be found that the core exhibits remanence but in the new direction of magnetic polarization. A suitable arrangement of permanent magnets and a means for changing the configuration or magnitude of the magnetic field which they impress on a core or body of soft magnetic material may be employed to accomplish the same purpose.

In many forms of electrical and electronic apparatus the speed with which the apparatus may function depends upon the time required for the reversal of magnetic polarization of core components. One example of such an apparatus is the digital computer, particularly that portion of the computer known as the memory array wherein information is stored by selectively directionally polarizing a great number of soft magnetic cores. As will be pointed out later, the speed with which this component functions governs, to a large degree, the speed of operation of the computer. One of the limiting factors in increasing the speed of operation has been the time required to reverse the magnetic polarization of the core elements without the attendant gross increase in power required by usual methods. It should be appreciated that the foregoing problem is not confined to digital computers, but extends to many other applications and apparatus where it is necessary to increase the speed of reversal of the magnetic polarization of cores made from soft magnetic material. Our invention is directed to the decrease of the time required for the reversal of the magnetic polarization of soft magnetic materials without a gross increase in power requirements. Further, by the practice of our invention, the operation of existing apparatus in which rapid reversals of the magnetic polarization of such soft magnetic cores is necessary may be rendered more efiicient because, with the same available power such reversals may be accomplished in a shorter time, or the reversals may be accomplished in the same time with the expenditure of less power.

In accordance with the foregoing, a principal object of our invention is the provision of electromagnetic apparatus capable of reversing the polarity of its magnetic components in a shorter time without grossly increasing the power required to accomplish this function.

A yet further object of our invention is the improvement in operating time of electromagnetic apparatus such as digital computers by reducing the time required to reverse the magnetic polarity of magnetic elements there- A still further object of our invention is the provision of an improved method of reversing the magnetic polarity of soft magnetic materials. Other objects and features of our invention will become apparent from the disclosure which follows.

Briefly stated, in accordance with one aspect of our invention we have discovered that the time required for the reversal of the magnetic polarization of soft magnetic materials may be significantly lowered by changing the configuration of the driving field from a square wave or step function to one in which a short duration pulse of high amplitude is immediately followed by a longer duration low amplitude wave shape. In practicing our invention in its broader aspects, it will occur to those skilled in the art that such a relationship between the short duration pulse of high amplitude and the longer duration low amplitude wave shape may take various forms and that the principles about to be disclosed are equally applicable to both metallic and non-metallic soft magnetic materials.

Our invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing FIG. 1 is a schematic representation of the direct current hysteresis loop of a soft magnetic matreial, FIG. 2 is a schematic representation of a soft magnetic core having energizing and output coils wound thereon, FIGS. 3 to 12 are graphical representations of magnetic fields applied to soft magnetic materials and the reaction of the materials thereto, FIG. 13 is a circuit diagram, FIG. 14 is a fragmentary schematic representation of a circuit employing soft magnetic cores, and FIG. 15 is a detailed schematic representation of a single core in the circuit shown in FIG. 14.

As is commonly known, soft magnetic cores for use in electrical and electronic apparatus may be made from such materials as silicon-iron, iron-nickel alloys, iron and other metals having soft magnetic properties, as well as from non-metallic soft magnetic materials such as, for example, ceramics of the spinel ferrite type. In some low power applications cores are formed from iron or the iron-nickel alloys rolled into thin tape and wound into a toroidal form. In others, cores are formed in a variety of shapes, including toroidal, from fused spinel ferrite ceramic. In still others, small particles of iron or iron-nickel alloy are insulated from each other by an inert substance and formed into a variety of shapes including toroidal.

In such applications as digital computer memory arrays as referred to previously, it is quite common to use toroids of soft magnetic material as cores. However, it should be realized that other and specifically different shapes or configurations of cores may be used equally well in apparatus of this type or in other analogous environments. The examples referred to subsequently in this disclosure and test results included for the exemplary cores have been obtained from toroidal shaped cores. It should, however, be understood that our invention is not limited to any specific configuration of core and that it may be advantageously applied to other and specifically different core configurations.

The magnetic behavior of cores made from soft magnetic material may best be understood by reference to FIGS. 1 and 2 of the drawing. FIG. 1 is a somewhat idealized reproduction of the direct current hysteresis loop of a representative square loop soft magnetic core. In the figure, the axis of the abscissa represents applied magnetic field, H, in oersteds and the axis of the ordinate represents the magnetic flux density or induction, B, in gauss. The symbols H and B for these quantities is according to conventional Gaussian notation.

As is well understood, and with reference to FIG. 1, when the field is increased in the direction of +H, the saturation of the core increases to a value of l-B and remains substantially at that value for further increases in H as indicated at +B Usually B has a somewhat higher value than B for these materials. When the field, i-H, is removed the saturation returns to +B and remains there. The residual saturation or magnetism is referred to as remanence and the magnitude of B reflects the degree of retained magnetic polarization in the material.

When a reverse field, H, is applied, the material resists reversing its direction of magnetic polarization until a reverse field having the magnitude H is reached at which field the direction of saturation passes through the zero value of B and reaches a value of B, and as the field is increased to higher values of H, the magnitude of the saturation attains the value shown at B When the field, H, is removed, the saturation value of the core returns to B which is equal in magnitude but opposite in magnetic polarization to +B As the field is increased toward +H the remanence or residual saturation B, remains substantially constant until a value of +H which is equal in magnitude to -H is reached. After a sufficient time at a field of l-H the saturation reverses to l-B and attains a value for fields in excess of +I-I as indicated at +B l A toroidal core of soft magnetic material 10 is illustrated in FIG. 2. A coil of wire is schematically illustrated at 11 provided with terminals 12. If a source of direct current of sufficient magnitude is applied to terminals 12, the core will become magnetically polarized in, for example, the direction indicated by arrow 13. When the circuit is broken the core will remain magnetized in this direction, the magnitude of its residual magnetism corresponding to i-B in FIG. 1. If a reversed direct current is now applied to terminals 12 after a measurable time interval the direction of magnetization of core 10 will reverse as indicated by arrow 14. When the current is interrupted the core will remain magnetized in this direction, the magnitude of its residual magnetism corresponding to B, in FIG. 1. Continued reversals of current and consequent reversals of the magnetic field produced in the core 10 by the coil 11 will cause the magnetic polarization of .the core to reverse substantially as shown by the typical hysteresis loop in FIG. 1. The magnetic characteristics of the core 10 and its reaction to current applied to coil 11 may be detected and measured by means of an output coil 15, to whose terminals 16 measuring and analyzing apparatus may be connected.

The hysteresis loop shown in FIG. 1 is not representative of any particular core material but has been drawn to illustrate the general configuration of many so-called rectangular or square-loop materials which are used for cores. The rectangularity of the loop is self-evident and this configuration is one which is highly desirable and, indeed necessary in some applications for cores of this type. It should be appreciated that individual cores of a nominally square-loop material may show varying degrees of rectangularity and that different materials have different intrinsic properties which may effect their relative rectangularity.

As is well known and as previously stated, there is a measurable elapse of time which is required to reverse the magnetic polarization of a soft magnetic core material which varies with the applied field. In general, there is a minimum steady field which corresponds to the coercive force or H of a given soft magnetic material which will cause reversal over a relatively long period of time. As the strength of the field is increased the reversal time is shortened. There is a practical limit to which the reversal time for any particular core may be reduced by simply increasing the field. To significantly reduce the reversal time by merely increasing the applied field causes the power requirement of the source generating the field to become prohibitive since, for a given size core, the number of turns of winding on the core has an upper limit and in order to increase the field applied to a core by means of a given number of core windings higher sustained currents are needed.

We have discovered that the time required to reverse the magnetic polarization of soft magnetic materials may be markedly decreased without the employment of significantly higher power as disclosed previously. This may be accomplished by changing the wave shape of the applied field, H, i.e., the time versus amplitude distribution of the reversing field. To more adequately explain our invention reference is made to FIGS. 3 through 12 of the drawing.

The reversal of magnetic polarization of soft magnetic cores as practiced prior to our invention has been accomplished as indicated by FIGS. 3 and 4. The solid line wave form in FIG. 3 illustrates a conventional steady state field of H oersteds in which H is greater than the coercive force, H of the core for a time interval represented by the expression T -T as applied to a magnetically polarized core to reverse its polarization. The time required to reverse the magnetic polarization by this field may be measured and plotted against the output voltage determined from, for example, coil 15 in FIG. 2, as shown in FIG. 4. In FIG. 4 the time interval for such reversal shown as a solid line is represented by the expression T -T which is identical to the time interval T -T in FIG. 3.

In order to decrease the reversal time interval T -T using a square-wave, substantially constant amplitude field as illustrated in FIG. 3, the amplitude of the field must be increased. This behavior is graphically illustrated by the dashed line curves in FIGS. 3 and 4. In FIG. 3, for example, the amplitude of the field H, is greater than that of H, and the time interval T T is significantly less than T T However, the power requirements and losses in a system capable of producing a sustained field having an amplitude H',, are much greater than the power requirements and losses of the same kind of system capable of producing a sustained field having an amplitude H rendering this manner of shortening the reversal time relatively inefficient. Since, by the well known laws of electromagnetism, the energy dissipated in the magnetic material is proportional to the product of the magnetic field and the change in magnetic polarization which is the same for both the longer and shorter reversal time intervals, the total energy dissipated in the magnetic material is greater for the shorter interval. For this reason both the total energy dissipation from the power source and heating of the magnetic material is greater for the shorter reversal time interval than the larger reversal time interval.

It should be noted at this point that the shapes of the curves shown in FIG. 4 are not intended to represent any particular core material but are merely exemplary and are only intended to illustrate in general the more or less characteristic behavior of soft magnetic cores during reversal. Different core materials may exhibit reversal curves which differ from those illustrated in FIG. 4 in specific, but for the purposes of this disclosure, nonessential details.

We have discovered that by changing the wave form of the field applied to a magnetically polarized soft magnetic body, the reversal time may be reduced significantly without the gross increase in the power requirements of the field supplying system required when an equivalent reduction in reversal time is accomplished by means of a rectangular wave form field as illustrated in FIGS. 3 and 4. This relationship between the wave form or shape of the applied field and the reversal time of soft magnetic materials may best be illustrated by comparing FIGS. 5 and 6 with previously discussed FIGS. 3 and 4.

The wave form in solid lines in FIG. 5 illustrates a short time duration pulse of high amplitude, H immediately followed by a longer duration lower amplitude field, H having a rectangular or square configuration. The core to which this field is applied has magnetic characteristics similar to the idealized material whose characteristics are illustrated in FIGS. 3 and 4 and the values shown for magnetic field strength, time interval and output voltage are equivalent. As shown in FIG. 6, the time interval required to reverse the magnetic polarization of the core using a wave form field in which the sustained field has an amplitude H illustrated in solid lines in FIG. 5, could be, for instance, about one-half that required to accomplish the same reversal using the square wave form field having an amplitude H illustrated in solid lines in FIG. 3. Further, as is Well known, because a high amplitude short time duration pulse field of the type illustrated may be produced without markedly increasing the average power requirements of the field generating system, the much shorter time required to reverse the core may be accomplished quite efiiciently, i.e., with a substantial saving in the power required to reverse the magnetization by the conventional square wave form field illustrated as the dashed curve in FIG. 3.

As furhter shown in FIG. 5, by employing a high amplitude short time duration pulse followed by a much lower amplitude square wave field H" reversal of the magnetic polarization may be accomplished in the same time interval Tg-Tl of FIG. 3 but with a possible saving in system power requirements.

A toroidal core was formed by winding a metal tape 0.005 inch thick by /2 inch wide of Permalloy, an alloy having a nominal composition of 65 percent nickel, balance iron, into a toroid having a means diameter of about 4 centimeters. There were 29 turns of tape in the core. The coercivity, H of this core was measured and found to be 0.01 oersted. The radial cross-sectional area of the toroid was about 0.0725 square inch. Two coils were wound upon the toroid as illustrated in FIG. 2. The energizing coil 11 was composed of 200 turns of wire and the output coil 15 was composed of 10 turns. A field of about 0.15 oersted was applied by connecting a constant current source to terminals 12 for sufficient time to produce magnetic polarization in the core after which time the current was removed. The output coil terminals 16 were connected through an appropriate circuit to a cathode-ray oscilloscope.

The magnetic polarization of the core was then reversed as follows. A constant current source having opposite polarity to the initial current previously applied was connected to terminals 12 to supply a field consisting of an initial pulse field of about 3.6 oersteds for about 10 microseconds followed by a steady field of 0.15 oersted for more than 10,000 microseconds. This field is schematically illustrated in FIG. 7 in which T T represents more than 10,000 microseconds. It is to be noted that the time axis of the graph is broken to enable a sufficiently large scale to be used to more clearly show the ideal configuration of the initial pulse.

A photographic record was made of the signal generated in output coil 15 and traced upon a cathode-ray oscilloscope screen. This is reproduced in solid line in FIG. 8. It will be seen that under the wave form field according to our invention the magnetic polarization of the core was reversed in an elapsed time of about 3600 microseconds.

The same experiment was repeated on the same core and with the same equipment except that a conventional square-wave form field was used to reverse the magnetic polarization. In this case a steady field of 0.15 oersted was maintained for more than 10,000 microseconds. The chain line curve shown in FIG. 8 is reproduced from a cathoderay oscilloscope trace of the signal obtained from output coil 15 and shows the rate of reversal of magnetic polarization. It will be seen that the conventional square wave form steady field requires more than twice the elapsed time, i.e., about 8400 microseconds, to reverse the magnetic polarization of the core compared to the wave form of our invention. Note also that the amplitude of the induced voltage observed, which is a measure of the speed of magnetic polarization change at any time is greater for the 0.15 oersted field when this field is preceded by the 3.6 oersted pulse than when this same 0.15 oersted pulse acts alone.

As will be apparent to one skilled in the art, this shortening of elapsed time for the reversal of the magnetic polarization can be accomplished with only a slightly greater average power requirement, since only a small fraction of the reversal of magnetic polarization took place at the higher field.

As stated previously, our invention may be equally advantageously used with soft magnetic ceramic cores. An example of our invention applied to a core made from such a material is illustrated in FIGS. 9 to 12. A spinel ferrite composed of about 43 percent Fe O 14 percent MgO and 43 percent MnO was formed into a toroidal core shape and fired. The resulting toroidal core had an outside diameter of about 0.1 inch, an inside diameter of about 0.05 inch and a radial cross-sectional area of about 0.0015 square inch. The direct current magnetic properties of this core were measured and it was found to have a coercivity (H of 0.8 oersted and a saturation flux density (B of about 1000 gauss.

The ferrite core was provided, as illustrated schematically in FIG. 2, with a five turn field or energizing coil 11 and a five turn output coil 15. A field of about 2.4 oersteds was applied by connecting a constant current source to terminals 12 for a suflicient time to produce magnetic polarization in the core after which time the current was removed. The output coil terminals 16 were connected through an appropriate circuit to a cathode ray oscilloscope.

The magnetic polarization of the core was then reversed as follows. A constant current source having opposite polarity to the initial current above was connected to terminals 12 to supply a square wave steady field of about 0.91 oersted for more than microseconds. This field is schematically illustrated in solid lines in FIG. 9 in which T -T represents more than 10 microseconds.

A photographic record was made of the signal generated in output coil and traced upon a cathode-ray oscilloscope screen. This is reproduced in solid line in FIG. 10. It will be apparent from this curve that the magnetic polarization reversal which this field produced in the ferrite core required about 7 microseconds and as will be understood by those skilled in the art was probably not complete. It should be noted, however, that the maximum voltage observed for this 0.91 oersted field alone was not greater than .2 volt. This is a measure of the speed of the changing magnetic polarization.

The same experiment was repeatd on the same core and with the same equipment except that a square wave field of about 2.02 oersteds or about twice the field previously applied was applied for more than 10 microseconds. This is schematically illustrated by the dashed line curve in FIG. 9 wherein T -T equals more than 10 microseconds. The dashed line curve shown in FIG. 10 e is a reproduction of the oscilloscope trace of the signal obtained from output coil 15 and illustrates the reversal of magnetic polarization of the core. It will be seen that under these conditions the magnetic polarization of the core was completely reversed in about 1.5 microseconds.

The same procedure was repeated with the same core and with the same equipment except that a field comprising an initial pulse field of about 2.02 oersteds for about 0.4 microsecond followed by a steady field of about 0.91 oersted for more than 10 microseconds was applied to the core. This field is schematically illustrated in FIG. 11 in which T T represents more than 10 microseconds. The curve illustrated in FIG. 12 is a reproduction of the signal generated by output coil 15 in response to this field and traced upon a cathode ray oscilloscope. It will be noted that the reversal of magnetic polarization of the core was accomplished in slightly over 2 microseconds. Furthermore, the speed with which the magnetic polarization changes (as measured by the induced voltage) under the influence of the 0.91 oersted field is greater when this field is preceded by the 2.02 oersted pulse, than when this 0.91 oersted field acts alone.

From the foregoing examples it will be apparent that our invention permits the reversal of the magnetic polarization of soft magnetic materials to be accomplished with greater speed with less increase in average power requirements than previously practiced conventional reversal techniques.

The increased efficiency with which our invention accomplishes the reversal of magnetic polarization in soft magnetic materials may be explained by the following probable theory.

The alternation of the energy of a physical system is commonly called a change of state. When such a change of state involves the gross modification of the physical, magnetic and electrical properties without changing the net chemical composition of a material, it is called a phase transformation. Some examples of phase transformations are the familiar changes of state which accompany the freezing, melting, boiling and condensation of a material such as water.

Phase transformations of an analogous nature occur in many other instances in addition to those mentioned above, for example, in magnetism. A general characteristic of a phase transformation is that a change occurs in the material transforming it from one stable state into another via a phenomenon termed nucleation.

In applying the principles of nucleation to the change of state of magnetic materials and more specifically to that change of magnetic state represented by the reversal of magnetic polarization of a body of magnetic material, assume a body of magnetic material which has a net magnetic polarization in a given direction and that this polarization is stable in the absence of external magnetic fields. Because of physical inhomogeneities throughout this body there are small fluctuations in the direction of polarization of a large number of regions within the body. The size of these regions of fluctuation varies from very small to comparatively large according to a type of statistical distribution. In order to reverse the net polarization of the body, it is necessary to apply a magnetic field having a direction opposite to the direction of the net polarization of the body and having a magnitude equal to or greater than the coercivity H of the body. For a given magnitude of an applied magnetic field greater than H only those regions of fluctuation larger than some critical size will begin to grow, these growth susceptible regions being termed nuclei. During the application of the reversing field, the regions which are smaller than the critical size do not grow while those of the nuclei size grow over a measurable elapse of time to occupy the entire volume of the body. Obviously, the elapsed time required to complete the reversal of polarization is a function of the number of nuclei, assuming a constant growth rate for the nuclei. It is postulated that the number of nuclei is determined by the amplitude of the field, the larger the amplitude of the field, the smaller the critical size of the region of fluctuation which may grow. It is further postulated that the number of nuclei can be increased by growing some of the sub-critical nuclei in a higher amplitude field for a time just long enough for them to become super-critical for a subsequently applied lesser field in which they will continue to grow. Therefore, for a given amplitude steady field preceded by a nucleating pulse field there are provided more growing regions in the body and the change of state is accomplished with greater speed than if the initial nucleating pulse were omitted.

In the pracitce of our invention, the amplitude and time duration of the nucleating pulse is preferably such that a substantial fraction of the total change in magnetic polarization of the body of soft 'magnetic material is accomplished. Further, reversal of magnetic polarization may be accomplished even if there is a slight hiatus or time delay after the application of the nucleating pulse and before the application of the lower amplitude, steady state field.

It has been found that the reversal of magnetic polarization in materials of this type may be accomplished with a steady state field having an amplitude less than the coercivity, H of the material by applying an initial field having an amplitude greater than the H of the material and maintaining this high amplitude field for a time sufficient to accomplish a substantial fraction, for example, about one-tenth, of the total change in magnetic polarization of the body, followed by a steady field having an amplitude less than H In general, the reversal of magnetic polarization of soft magnetic materials may be more efficiently accomplished according to our invention by the application of a nucleating pulse field having an amplitude at least ten percent greater than the amplitude of the steady state field which follows. The duration of the nucleating field should usually not exceed about one-fourth the time necessary for reversal by a steady state field of magnitude equal to that of the nucleating field acting alone.

An example of a circuit capable of providing a pulsed field as previously described to a soft magnetic core is illustrated in FIG. 13. A pair of pulse generators 20 and 21 are each connected to the control electrodes 22 and 23 of double triode tube 24 hereafter referred to as the mixer. The pulse generators 20 and 21 are conventional commercially available apparatus and therefore will not be described further. The pulse generated by each generator is graphically described by the potential E versus time graphs associated with each generator. It is to be noted that the polarity of the pulses are indicated by the direction of the arrowhead on the E axis which is opposite to ground potential.

The anodes 25 and 26 of the mixer 24 are connected to each other and to a source of constant voltage indicated by the symbol +B through a resistor 27 and to one side of a capacitor 28. The other side of capacitor 28 is connected to a source of constant voltage indicated by the symbol --E through a resistor 29 and to the control electrode 30 of triode 31, hereafter referred to as the driver. The anode 32 of the driver is connected to a source of constant voltage indicated by the symbol +B and the cathode 33 of the driver is connected to ground through energizing coil 34. Coil 34 is adapted to apply a magnetic field to core 35.

In operation, the pulse generators are synchronized to start delivering their respective pulses simultaneously. When there is no voltage delivered from either pulse generator 20 or 21, mixer 24 draws heavy current from the -+B supply, the particular value of the current being determined by the characteristic of the driver tube 24. No signal will appear on the grid of the driver tube 24 because the voltage is constant and therefore there will be no flow of current across the capacitor 28. The value of E is sufficient to prevent any flow of current through the driver tube and therefore there can be no flow of current through the coil 34.

When the pulse generators 20 and 21 simultaneously produce pulse signals as indicated, the control electrodes 22 and 23 are correspondingly energized and decrease the current flowing across the mixer 24. The voltage appearing at the capacitor 28 via resistor 27 increases in response to the signal impressed on the mixer control electrode and has the same wave shape as the composite generated pulse signals. The capacitor 28 passes the varying voltage signal which appears at the control electrode of the driver 31 as positive voltage of sufficient amplitude to permit current flow through the driver 31 to energize coil 34. The wave shape of the current flowing through coil 34 and consequently the wave shape of the field applied to the core 35 is proportional to the wave shape of the composite voltage signal applied to the control electrode of driver 31, as indicated schematically by the I versus time graph. It will be appreciated by those skilled in the art that other and specifically different circuits may be devised within the skill of the art to produce the desired field applied to core 35 and that the circuit shown in FIG. 13 is merely exemplary.

As previously stated, one application to which our invention may be advantageously applied is in the improvement in operating speed of digital computers. In many digital computers one of the fundamental components is a memory unit wherein information is stored for use in operations that follow a predetermined sequence or program. In a widely used type of memory system-the coincident current type-the information is stored in an array of ceramic or metallic magnetic cores which have rectangular hysteresis loops. The information is stored in the form of yes or no type signals sometimes referred to by symbols such as and 1 and the information is employed during the operation of the computer by application of a binary mathematic system of symbolic logic to provide a finite result. The scheme by which these arrays function is well known and will only be reviewed in terms of operations on a single core representative of the basic operation of the array.

A schematic representation of a portion of a typical memory array is illustrated in FIG. 14 in which the small circles 40 are schematic representations of toroidal soft magnetic cores. The grid or network of lines y y and 3 and x x and x are schematic representations of current conducting lines supplying power to energizing coils on each core. The undulating line 1 passing from core to core in FIG. 14 is a schematic representation of an electrical conductor connecting sensing coils which are also wound upon the individual cores. An individual core 40 with its respective windings is shown in FIG. 15 in which coil 41 represents a coil energizable by one of the y conductors. Coil 42 represents a coil energizable by one of the x conductors and coil 43 represents a sensing coil connected to the z conductor.

The speed of operation of such a memory array is limited by the speed of operation of a single core as will be presently explained. The information is stored in the binary system in terms of the state of magnetization of the cores. For example, and with particular reference to FIG. 1, a core which is in the state 0 has its magnetization in the stable state indicated by B and for the "1 state its magnetization is stable at +B This information is obtained from a core (x y for example, by simultaneously passing through the windings supplied by x and y currents whose fields are additive in the core and are in the direction of +H. The magnitude of the separate fields are each equal to some value H,,, where H, is less than H for the core and greater than H /Z. It will be readily appreciated that when a single signal of such magnitude acts alone on a core whose state is 0, the magnetization is essentially unchanged but when two signals act simultaneously and additively upon the core, the state of the core will be changed from 0 to 1, i.e., its magnetic polarization is reversed, with a resulting voltage due to the change of magnetization observed in the sensing winding. The speed with which the magnetization changes is determined by the amplitude of the field in excess of the value 1-1,, for a given core. Consequently, in this present conventional technique, the maximum switching speed obtainable with conventional rectangular wave, constant amplitude fields is accomplished with a total field having an amplitude slightly less than twice H assuming an ideal rectangular hysteresis loop core material. The same technique described for reading out the stored information is employed in the writing in operation and so the overall operating speed of the system is limited by the speed of a single reversing operation on any one core.

In the usual type of memory array, the x and y fields are superimposed only on one core at any one time but the separate fields of x and y are applied upon all cores on the selected x coordinate and on the y selected coordinate. In the application of our invention to such an array, the nucleating pulse which precedes the steady field pulse or signal from a single energizing coil may be larger than H, of the core and will grow many nuclei but the duration of the pulse is sufficiently short so that when it is removed and the amplitude of the field drops to the steady state value (which is less than H but greater than H /2), these nuclei will collapse to much smaller sizes, hence accomplishing no appreciable net reversal of the magnetization of the core. The voltage observed when these nuclei collapse is small compared to the voltage due to a core reversal and consequently discrimination against this noise poses no great problem. On the other hand, when two such signals are simultaneously imposed upon a core and are additive, the amplitudes of the resulting nucleating pulse and of the steady field following the nucleating pulse are approximately twice that of a single signal. Under such circumstances, as previously shown, the time required for the reversal of the magnetic polarization of such a core is significantly reduced.

From the foregoing description, it may be seen that we have described certain preferred embodiments of our invention. However, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. In a memory array circuit adapted to be used with a digital computer, a magnetically polarized soft magnetic core, a plurality of coils wound upon the core, means for simultaneously energizing at least two of the coils whereby the magnetic fields produced by each energized coil have a direction opposite to the polarization of the body and contribute to each other to produce a resulting magnetic field in the body, and means for controlling the time versus amplitude variation of the resulting field to provide an initial amplitude of applied field greater than the coercive force of the core for a period of time sufficient to cause about one-tenth of the core material to assume a reverse magnetic polarization and a substantially constant magnetic field of sutficient amplitude and duration to cause the remainder of the core material to reverse magnetic polarization.

2. An electromagnetic apparatus comprising a body consisting essentially of a soft magnetic material in an initial magnetically polarized state, a first means including an energizable electrical current comprising at least one electrical conductor adjacent to the body adapted to produce a magnetic field within the body having a direction opposite to the magnetic polarization of said body and having an ampitude greater than the coercive force of said body, and means for controlling the magnetic field produced by the first means so that the magnetic field produced within the body comprises an initial high-amplitude field greater than the coercive force of said body and of short time duration followed by a lower substantially constant amplitude, longer time-duration field, controlling means comprising pulse generating means, a triode mixer tube, the pulse generated by the generating means being supplied to the control electrodes of the triode, means for applying a positive potential to the anode of the triode and to a first terminal of a capacitor, means for applying a negative potential to a second terminal of the capacitor and to the control electrode of a second triode, means for applying a positive potential to the anode of the second triode and conducting means for transmitting electrical current from the cathode of the second triode to the electrical conductor adjacent to the body.

3. An electromagnetic apparatus comprising a body of soft magnetic material in an initial magnetically polarized state, and means coupled to said body for applying a magnetic field thereto having a direction and wave shape and time duration operative to effect complete reversal of magnetic polarization, the wave shape of the applied magnetic field during the time required to achieve complete reversal of magnetic polarization of the body comprising a first portion of amplitude greater than the coercive force of the body and of duration less than the time required for reversal of magnetic polarization of the body, and a second portion of lower amplitude and longer duration than the first portion following said first portion and continuing until magnetic polarization of the said body is completed.

4. An apparatus for reversing the magnetic polarization of a body of soft magnetic material from a state of remanent magnetization of one polarity to the state of remanent magnetization of the opposite polarity comprising means coupled to said body for producing therein a magnetic field having a first portion and a second portion, the first portion exceeding the coercive force of said body and terminating prior to complete reversal of magnetic polarization of said body and being of duration no greater than one-quarter as long as necessary to accomplish complete reversal of magnetic polarization of the body and be ing applied over the body coextensively with the second portion, and the second portion being of lower substantially constant amplitude and continuing after the termination of the first portion and until complete reversal of magnetic polarization of the body has taken place.

5. An apparatus for reversing the magnetic polarization of a core of soft magnetic material from a state of remanent magnetization of one polarity to the state of remanent magnetization of the opposite polarit comprising means including a coil wound around the core for producing therein a magnetic field having a first portion and a second portion, the first portion exceeding the coercive force of said core and terminating prior to complete reversal of magnetic polarization of said core and being of duration no greater than one-quarter as long as necessary to accomplish complete reversal of magnetic polarization of the core, and the second portion being of lower substantially constant amplitude and continuing after the termination of the first portion and until complete reversal of magnetic polarization of the core has taken place.

6. An electromagnetic apparatus comprising a body of soft magnetic material in an initial magnetically polarized state, a first means adjacent the body capable of producing a. magnetic field having a direction opposite the magnetic polarization of said body for causing a complete reversal of magnetic polarization thereof, and a second means for controlling the magnetic field produced by said first means so that the magnetic field produced in said body comprises a first portion and a second portion, said first portion having an amplitude exceeding the coercive force of said body and being of short time duration compared to the total period required to achieve said reversal of magnetic polarization of said body, and said second portion having a lower substantially constant amplitude and being initiated after an interval following termination of the first protion and continuing for the remainder of the period required for reversal of magnetic polarization of the body.

7. An electromagnetic apparatus comprising a body of soft magnetic material in an initial magnetically polarized state, a first means adjacent said body capable of producing a magnetic field within the body having a direction opposite the magnetic polarization of the body to etfect a complete reversal of the magnetic polarization thereof, and a second means for controlling the magnetic field produced by said first means so that the magnetic field produced within the body comprises an initial high amplitude field greater than the coercive force of the body and of time duration less than the period required to achieve complete reversal of magnetic polarization of said body followed by a field of amplitude less than the coercive force of said body and continuing through a time interval after termination of said initial high amplitude field until reversal of magnetic polarization of said body has been completed.

8. In a magnetic device, the combination of a magnetic element characterized by ditferent states of substantial remanence, and means for applying magnetizing forces to said element to change its state of remanence, said force applying means including means for applying to said element a large magnetizing force of magnitude in excess of the force required to change its remanent state and sufiicient to initiate reversal of magnetization of said element throughout its volume and for a time less than the time required with that large force to change its remanent state and for applying a smaller force after the cessation of said large force for a time sufficient to change said remanent state.

9. The method of switching from one stable state of remanent magnetization to another stable state of remanent magnetization to another stable state of remanent magnetizaion a body of soft magnetic material having a substantially rectangular hysteresis curve characteristic and having a winding operatively associated therewith com prising the steps of generating a current pulse having a steep leading edge and substantially constant amplitude, said current pulse constant amplitude being of sutficient magnitude to effect switching of said magnetic body from one stable state of remanent magnetization to another stable state of remanent magnetization, altering said pulse to provide an initial overshoot of short time duration and having an amplitude of at least one-fourth the amplitude 13 of said current pulse thereon, and applying said altered current pulse to the said winding.

10. The method of switching from one stable state of remanent magnetization to another stable state of remanent magnetization a body of soft magnetic material having a substantially rectangular hysteresis curve characteristic and having a winding operatively associated therewith comprising the steps of generating first and second current pulses each having steep leading edges and substantially 14 point in time, the duration of said second pulse being small compared to the duration of said first pulse, the amplitude of said second pulse being at least one-fourth the amplitude of said first pulse, combining said first and second pulses to produce a composite current wave elfectively having an initial overshoot, and applying said composite wave to said winding.

No references cited.

constant amplitude, said pulses originating at the same 10 JAMES W. MOFFITT, Primary Examiner 

